Could you give some preferably simple or very simple examples which show the power of LCSs?

another demo, somewhat more complicated. All parametric of course. I don't see how this could be done without LCS, with Datum Planes may-be, but then you don't see which way the plane is oriented, which is actually the subject of this thread.

Looks like an OCC bug: I tested my demo file and yours, and mine always works on OCC 7.3, and never on anything else, and yours always works on anything except 7.3

Yes, I think we are going to have a hard time on transition from older OCC to 7.3. It is not that 7.3 is buggy, but it actually fixed some bug in previous versions, but as a result (somewhat) breaks backward compatibility of the index based topological name. For example, see here and here.

Try Assembly3 (latest version 0.9.1) along with my custom build of FreeCAD at here.
And if you like to show your support, you can find the donate button at here.

Each section sketch can be attached directly to Sketch exactly like the LCS and edited parametrically. A sketch has its own 'LCS' defined by its attachment properties even though the axes are not visible after the sketch is closed. So, in this instance, the LCS's seem redundant.

What advantage am I missing again other than the visibility of the LCS?

Zolko, thanks for showing these examples. LCSs have what I had missed with the DatumPLanes: they show where the origin is. Besides that, their function is pretty similar. There are some advantages for LCS - such as direct attachments to XZ and YZ planes - and some for DatumPlanes - such as visualizing the plane. DatumPlanes offer the additional attachment modes indirectly by rotating the attached items by 90°.

What I really would like to see is a combination of both: An LCS having the three principal planes like the Body's Origin. They would have the same visibility behaviour and it should be able to select a plane and directly attach a sketch to it. An additional property could make the axis cross disappear.

I think the original request is a nice idea. Just adding a small axis cross to the visalisation of the datum plane would help a lot. One could put it on a corner vertex of the plane, this ensures it is always visisble (as the plane autoscales to be a bit larger than the largest model element). Or maybe just add two charchters "X" and "Y" on the side of the datum outside the rectangle, and an arrow on each vertex showing into Z. Would also make the orientation clear. A bit like Yoriks arch section views (I think they are called like that)

What I really would like to see is a combination of both: An LCS having the three principal planes like the Body's Origin. They would have the same visibility behaviour and it should be able to select a plane and directly attach a sketch to it. An additional property could make the axis cross disappear.

Something like these ?

LCS_AxisCross.png (203.08 KiB) Viewed 322 times

The App::Placement is defined visually in ~/src/Gui/ViewProviderPlacement.cpp

The Axis Cross is defined visually in ~/src/Gui/SoAxisCrossKit.cpp

The LCS is defined visually in ~/src/Mod/PartDesign/Gui/ViewProviderDatumCS.cpp

Personally, I prefer the behaviour of the Axis Cross: it has a defined size as per screen reference, meaning that when zooming in and out, it keeps the same size on the screen, doesn't follow zoom.

What advantage am I missing again other than the visibility of the LCS?

For example if you want to attach more than 1 object (sketches, axis...) on the same position: this took me 30 seconds (really)

LCS_demo6.2.png (315.73 KiB) Viewed 314 times

Another thing is, if you mess things up (because shit happens): if your sketches are attached to LCS, you'll have to re-attach the LCS-s but the sketches themselves will remain consistent. For example, in the demo6 attached previously, add another arc and another section to the Sketch profile, to make the sweep longer. It'll completely confuse things. Now, you can re-attach the LCS to the correct positions, add another LCS, another hexagonal sketch, re-edit the sweep to add those objects, and you're done. All the while dynamically seeing what you're doing.

But anyway, I don't have any incentive to convince you to use LCS or not, please do as you wish.